An optical resonator is an arrangement of optical components that enable a beam of electromagnetic radiation to circulate in a closed path. Optical resonators are a major component of lasers, surrounding a gain medium and providing feedback for laser beam generation. For example, an axial beam of electromagnetic radiation continues to build as it bounces back and forth across the gain medium disposed within an optical resonator. This accounts for the degree of coherence of the output laser beam. Although the gain medium amplifies the wave, the feedback provided by the optical resonator aids in building up a coherent laser beam. Optical resonators have also been adapted for use in optical parametric oscillators and interferometers.
Electromagnetic radiation of appropriate wavelengths bounces back and forth between the mirrors of an optical resonator and takes on a standing-wave configuration determined by the separation distance, L, between the mirrors. This distance L is called the “cavity length.” When the mirrors are made of an electric conductor, the optical resonator resonates (i.e., standing waves exists within it) when there is an integer number of half wavelengths spanning the cavity between the mirrors. In other words, electromagnetic radiation resonates in a resonator when
  L  =            m      ⁢                          ⁢              λ        m              2  where λm is the wavelength of the electromagnetic wave that can resonate in the optical resonator. In principle, there are an infinite number of possible oscillatory longitudinal resonator modes, each with a distinctive resonance wavelength λm.
FIG. 1 shows a side view and schematic representation of an exemplary optical resonator 100 comprising mirrors 102 and 104. The mirrors 102 and 104 form a resonant Fabry-Perot cavity with a cavity length L. Curves 106-108 represent three standing waves with resonance wavelengths λ1, λ2, and λ3, respectively, of electromagnetic radiation resonating within the cavity of the optical resonator 100. Curve 106 represents the longest resonance wavelength λ1 the resonator 100 can support and is called the “fundamental.” In other words, λ1/2 is the minimum cavity length L the resonator 100 can be configured with to provide resonance for the fundamental wavelength λ1. Curve 107 is the second longest resonance wavelength λ2 the resonator 100 can support, and curve 108 is the third longest resonance wavelength λ3 the resonator 100 can support.
In recent years, various types of optical resonators have been developed, such as photonic crystal resonators and ring resonators. However, for nearly all of these resonators, the minimum cavity length is about half the fundamental wavelength. Optical resonators that can be configured with sizes and cavity lengths that are smaller than half of the fundamental wavelength are desired.